Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
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-Section 45
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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Significant agreements - Deferred revenue (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred revenue
Deferred revenue	$ 34,253	$ 5,657	$ 14,635
AstraZeneca
Deferred revenue
Deferred revenue	4,658	$ 4,913	$ 4,727
AstraZeneca \| Target 3, Target 4, Target 5 and Target 6 Material Rights
Deferred revenue
Deferred revenue	$ 4,700